1. Field of the Invention
This invention relates in general to the post welding heat treatment of the welds of tubular members. It is especially suitable for increasing the strength in the weld and the heat affected zone to produce an overall strength that at least matches that of the body of the tubular member.
2. Description of the Prior Art
It is common to use electrical resistance (flash-butt) welding to join tubular members such as drill pipe and their rotary connectors, called "tool joints," in the oil well drilling industry. It is important that the weld and the heat affected zone be able to carry as high a tensile load and be able to withstand fatigue stresses or impacts as well as the pipe used in the assembly. Otherwise, the weld or the heat affected zone may not have adequate load carrying capability and may fail when subjected to the severe stresses commonly encountered during oil well drilling.
With reference to FIG. 5, a drill pipe A usually has an area B of increased cross-section on each end called an upset, which is welded at C to a tool joint D. It is possible to have a lower yield strength of the metal in the heat affected zone of weld C than in the body A of the pipe and still have greater overall strength in the weld and heat affected zone. This is due to the larger cross-sectional area in upset B as compared with the smaller cross-sectional area of the non-upset portion or body A of the pipe. Typical ratios of an upset B cross-sectional area to the pipe body A cross-sectional areas may range from 1.38 to 2.10 for commercially available assemblies. In some of the high strength pipe assemblies the load carrying capability of the heat affected zone around the weld C does not compare favorably with that of the pipe or tool joint when utilizing the prior art heat treatments such as normalizing and tempering the weld. In one prior art example, a 31/2 inch diameter 15.50 pound, S-135 drill pipe had a yield strength of 89,900 psi in the heat affected zone of upset B that was heat treated by normalizing and tempering, whereas the body A of the pipe had a yield strength of 140,000 psi. The ratio of the cross-sectional areas of the upset portion B of the pipe to the body A of the pipe was 1.42 to 1. With the pipe having a yield strength of 140,000 psi, a minimum yield strength of 99,000 psi is required in the heat affected zone of the upset B to essentially match the load carrying capability of the pipe. Using a safety factor of 10%, then the minimum yield strength of the heat affected zone should be at least 110,000 psi. The prior art method of normalizing and tempering the heat affected zone has been found incapable of producing such a yield strength in the heat affected zone.
The problem is more difficult than simply increasing the hardness of the weld and heat affected zone. Hardening too fast leads to the formation of cracks. Further, excessive hardness and brittleness leads to failures.